The present invention relates to a method for the formation of a protective coating film on the surface of a substrate material. More particularly, the invention relates to a method for the formation of a uniform protective coating film having a very high hardness on the surface of various kinds of substrates such as electronic parts and devices.
As is well known, the surface of various kinds of electric and electronic parts such as liquid-crystal display panels and the like is required to be protected against scratches and other mechanical damages by forming a protective coating film thereon having high hardness.
Liquid-crystal display panels usually have a structure consisting of two substrate plates made from a transparent material such as glass, acrylic resins, polyethylene terephthalate resins and the like held in parallel with a narrow gap space therebetween, which is filled with a liquid crystal substance, the peripheries of the parallel plates being air-tightly sealed with a glass frit or an organic sealant. Each of the two parallel plates is provided on the inwardly facing surface with a pattern of electrodes while an orientation membrane is formed on the areas in contact with the liquid crystal substance which serves to align the liquid crystal molecules in a definite direction when an electric voltage is applied to the electrodes. Such an orientation membrane is prepared by forming a coating layer of an organic synthetic resin on the surface of each of the substrate plates having the electrodes followed by rubbing the surface in a definite direction with a cotton cloth and the like, i.e. a so-called rubbing treatment.
In the above described manufacturing procedure of liquid crystal display panels, however, a trouble is sometimes unavoidable that cracks are formed in the electrodes under the influences of mechanical forces. This is presumably a consequence of the difference in the thermal expansion coefficients which is not negligible between the orientation membrane made from an organic material and the electrodes made from an inorganic material of high hardness such as ITO and the like.
With an object to solve the above mentioned problems, a method is proposed according to which a protective coating film mainly consisting of silicon dioxide is interposed between the electrodes and the orientation membrane. Such a silicon dioxide-based intermediate film can be prepared by coating the substrate surface with a liquid coating composition containing a silanol compound followed by baking of the coating layer while such a baking treatment causes another problem that degradation is caused in the electrodes to increase the electric resistivity thereof because the baking temperature must be as high as 400.degree. C. or even higher.
In recent years, moreover, so-called plastic liquid crystal display panels have been developed, of which the substrates are made from a synthetic resin such as acrylic resins, poly(ethylene terephthalate) resins and the like. It is important in such a plastic liquid crystal display panel that the electrodes are provided with a protective film having a high hardness and capable of being formed by baking at a relatively low baking temperature because the plastic substrates are thermally not stable enough at an elevated temperature.
Another important objective in the electronic industry, of which a high-hardness protective film is desired to be formed on the surface, is a color filter which is a transparency colored pattern-wise mainly in three primary colors of red, blue and green and used by mounting on the substrate plate of a color display device such as a liquid crystal color display panel. In the preparation of such a color display device, the preparation of the color filter is followed, usually, by the formation of a transparent electroconductive film and an orientation membrane.
Troubles are sometimes unavoidable in this procedure that creases and cracks are formed in the colored coating film of the color filter or fissures are formed in the electroconductive film. These troubles are caused as a consequence of the larger than negligible difference in the thermal expansion coefficients between the colored coating film, which is made from an organic material such as dyes, organic pigments, synthetic resins and the like, and the transparent electroconductive film made from an inorganic material of high hardness such as ITO and the like.
Various proposals have been made heretofore to solve the above mentioned problems, for example, by providing a protective film on the colored coating film of the color filter. Examples of such a protective film include those formed by coating the surface with a solution prepared by dissolving a solvent-soluble polyamide resin or a polyimide resin in a lactone compound, a phenol compound or a mixture thereof as the solvent followed by drying as disclosed in Japanese Patent Kokai 62-163016, those consisting mainly of silicon dioxide as disclosed in Japanese Patent Kokai 62-242918 and those consisting of an overcoating layer of a thermosetting resin which can retain transparency by curing, of which the temperature for the exothermic peak in the crosslinking and curing reaction is 200.degree. C. or higher as is disclosed in Japanese Patent Kokai 63-131103. These protective films heretofore proposed, however, are not quite satisfactory in respects of the heat resistance, light fastness, hardness, adhesion and stability against sputtering so that it is eagerly desired to develop a method for forming a protective film on a color filter, of which high precision is essential, without problems due to the above mentioned deficiencies in the prior art methods using conventional coating agents.